1. Field of the Invention
The invention relates to a method and a device for acquiring and processing signals obtained in wells and particularly in substantially horizontal wells or drill-holes passing through a subterranean zone, giving a better discrimination between the discontinuities of the sub-soil. The invention applies particularly well for discriminating between geological discontinuities oriented substantially in the same direction as the well and situated on each side thereof, from logs obtained by means of a well-logging tool moved in the well.
2. Description of the Prior Art
The investigation of a zone of the sub-soil likely for example to contain hydrocarbons may be achieved by drilling a well through the zone and lowering thereinside a well-logging tool comprising one or more transmission units adapted for transmitting signals into the geological formations around the well and one or more reception units for receiving the signals reflected and refracted by the discontinuities of the sub-soil more or less close to the well. The signals received are generally transferred to a surface laboratory and recorded. The recordings are then processed so as to make them readable by an interpreter. The signals transmitted and received may, be of very different kinds. They may, for example, acoustic waves or electromagnetic waves, etc. The method of the invention will be described hereafter using acoustic waves but it goes without saying that this is only one particular case which does not restrict the generality of the method.
Drill-holes are often vertical or substantially, so but in the zones of the sub-soil which geologists consider the most likely to contain hydrocarbons, it is usual to drill holes which are very highly slanted with respect to the vertical, and even substantially horizontal, so as to pass through the subterranean layers at a small angle and obtain information about the superincumbent bed and the floor (or base) of a possible reservoir. Under these conditions, it is important to discriminate between the waves received from formations above and below the drill-hole. That is possible so long as the dip of the reflectors with respect to the direction of the well is not too small and is of the same sign towards the top and the floor of the zone passed through or reservoir. If such is the case, the apparent speeds of the acoustic signals received (events) on each side of the well are of opposite signs and can be distinguished using a conventional apparent speed filtering method, as is well known in geophysics.
A prospecting method applicable in wells which are slanted slightly with respect to the horizontal is described, for example, in the European patent application EP 246 148.
Discrimination in the well logs of signals coming respectively from the floor and from the top of a reservoir becomes very difficult when the relative dip of the reflectors with respect to the direction of the well is substantially zero, or reduced to a few degrees, because these signals have very closely related apparent speeds.
This configuration, which is frequent in very deviated or horizontal wells, is also found in the vertical or slightly slanted portions of the drill-holes passing through a zone of faults having substantially the same orientation. It is also known that the periphery of a well, whatever its direction, is generally adversely affected by the mud which was used during drilling and has infiltrated into the formations. The propagation speed of the waves in this peripheral zone is for this reason different from that in the surrounding formations, and parasite reflections occur from the interfaces substantially parallel to the direction of the well. Here again, discrimination in the logs between the reflected waves coming respectively from the opposite sides of the well is made practically impossible.